In recent years, solid state light emitting devices such as light emitting diodes (LEDs) have been developed as a type of energy efficient sources for industrial processes, for example, photoreactive or photo-initiated processes, such as photo-curing of inks, adhesives and other coatings. Traditional arc lamps, which are conventionally used as ultraviolet (UV) light sources for industrial processes, contain mercury. Thus solid-state light sources may be preferred to arc lamps for environmental reasons, as well as for having a longer lifetime. UV LEDs have attracted a lot of attention because they generate much less heat and consume much less power than arc lamps, while providing the same light output. Many inks, adhesives and other curable coatings have free radical based or cationic formulations which may be photo-cured by exposure to UV light. Applications for UV LEDs include curing of large area coatings, adhesive curing, as well as print processes such as inkjet printing. Curing uniformity is critical for many large area photo-induced curing processes.
In many UV photo-curing applications large areas must be illuminated with a high density of UV radiation. UV LED sources commonly used in the inkjet industry have lines or arrays of a large number of LEDs packed closely to each other so that jetted ink layers receive continuous irradiation. A typical LED based light source includes an array chip/die having many LEDs in order to achieve the energy density required to initiate the photochemical reaction. To efficiently build an LED array system with different length or area, such as a three inch, six inch or twelve inch length, usually a short LED array is built as a basic element, for example, one inch long to three inches long.
A single LED chip/die generally will not meet the application requirements for power and irradiance so it is common to combine multiple LED dies on a substrate, within manufacturing limitations, to form a multi-chip LED module. The large LED array system is built using multiple basic elements. These basic element arrays are fabricated on a substrate, which has the individual die bonded or soldered on printed circuit boards (PCB) in serial, parallel or a combination of serial and parallel. Large LED array light sources of a desired area can then be formed by combining many multi-chip LED array modules.
The traces of a PCB may be configured for high driving current, spacing for wire bonding, and/or edge clearance of high current PCB to meet the electrical PCB design standard, all of which require a minimum edge around the PCB. In the abutting region where two multi-chip LED array modules are joined together, a high density chip-to-chip spacing cannot be maintained for chips with uniform LEDs having uniform spacing. When these LED arrays are abutted, there is a gap or spacing between adjacent groups of LED arrays, which may be, for example, 2 to 4 mm (see FIG. 2). This means that there is uniform illumination intensity along the length of each module, but there is a dip in intensity in the region where each module abuts, which tends to cause a banding effect in the substrate being cured. This results in a decrease in the irradiance over the abutting region, thus the overall uniformity of the illumination area is compromised.
It is well known in the art that light extraction from an LED die can be improved by encapsulating the LED in a hemispherical dome or plane-convex lens comprising optical materials with an index of refraction greater than one. This dome or lens structure can also change the light directivity from the LED die.
U.S. Pat. No. 8,581,269 provides a non-evenly spaced LED array light source having a plurality of LED modules, each module comprising a module substrate carrying a plurality of LED light source elements arranged in an array, each module having at least one edge portion of the substrate abutting that of another module, and the spacing of LED light source elements of the array in each module being arranged to provide a higher density of die at edges of the array where edge portions of two modules abut. Thus, arrangements of LED die in each LED array provides for a substantially uniform irradiance where two modules abut, and reduce or overcome edge effects. U.S. Patent Publication 2013/0187548 A1 proposes a similar concept as U.S. Pat. No. 8,581,269.
LED encapsulation is widely used to increase light extraction from LEDs and to provide better light directivity, and to further provide better coupling from an LED die to curing targets. Sometimes, multiple layers with different refractive index and hardness are used to extract more light from LED Dies and to protect the LED wire bonding, as per, for example, U.S. Pat. No. 7,798,678 B2 and WO 05043598 A2. To achieve greater extraction efficiency, certain encapsulation dimensions and spacing between LEDs may be desirable for array encapsulation. For example, the diameter of an encapsulation lens should be greater than twice the LED dimensions. The spacing between LEDs should be greater than a diameter of the encapsulation. Because of these reasons, the above described non-evenly spaced LED array light source intended to improve the uniformity of illumination is not efficient for an encapsulated LED array.
In addition, for applications using a single LED array, the illumination levels may roll off rapidly in the area above the edges of the array (see FIG. 1D). Therefore, there is a need in the industry to overcome some or all of the abovementioned shortcomings.